The operation of expansion machines, e.g. steam turbines, and for instance with the aid of the Organic Rankine Cycle (ORC) method for the generation of electric energy using organic media. e.g. organic media having a low evaporation temperature which usually have higher evaporation pressures at same temperatures as compared with water as working medium, is known in the state of the art. ORC plants represent a realization of the Clausius Rankine Cycle in which electric energy is generated, for instance, basically through adiabatic and isobaric changes of state of a working medium. Mechanical energy is generated by the evaporation, expansion and subsequent condensation of the working medium, and is converted into electric energy. Basically, the working medium is brought to an operating pressure by a feed pump, and energy in the form of heat, which is provided by a combustion or a flow of waste heat, is supplied to the working medium in an evaporator. The working medium flows from the evaporator through a pressure pipe to an expansion machine where it is expanded to a lower pressure. Subsequently, the expanded working medium steam flows through a condenser in which a heat exchange takes place between the vaporous working medium and a cooling medium. Then, the condensed working medium is recirculated by a feed pump to the evaporator in a cycle.
One particular class of expansion machines is represented by volumetrically working expansion machines, which are also referred to as displacement expansion machines, which comprise a working chamber and which work during a volume increase of this working chamber as the working medium expands. These expansion machines are realized, for instance, in the form of piston expansion machines, screw expansion machines or scroll expanders. Volumetrically working expansion machines of this type are used in particular in low performance class ORC systems (e.g. electrical power of 1 to 500 kW). As opposed to turbines, however, volumetrically working expansion machines require lubrication by means of a lubricant, in particular of the piston and the profiles of the expansion chamber rolling upon one another, and of the rolling bearings and the gliding walls of the working chamber. Hence, it is necessary to lubricate the bearing surfaces and the contacting flanks. The use of a lubricant advantageously also leads to a sealing of the working chamber of the expansion machine, so that less steam is lost by an overflow inside the expansion machine and the efficiency is increased. A lubrication with oil is advantageous, with oil and live steam passing the expansion machine at the same time, which necessitates a subsequent separation of the oil and the steam.
A method and a device for the lubrication of volumetrically working expansion machines are described in the European patent application No. 11000329.0, which represents internal state of the art by the applicant of the present invention.
This lubricating system is schematically shown in FIG. 1. It comprises in one example a lubricant separator (e.g. an oil separator) 10 which is connected between an evaporator 20 that supplies a completely or partially evaporated working medium and an expansion machine 30 that cooperates with a generator 40 and serves in the generation of electric energy. In this design, at least a portion of the lubricant is separated from the live steam of the working substance mixed with the lubricant which is supplied to the expansion machine 30. In the oil separator 10 corresponding separating sheets may be provided such that the working medium arriving at the expansion machine 30 still contains a sufficient quantity of lubricant (lubricating oil), so that it is possible to achieve a reliable lubrication of parts of the working chamber of the volumetrically working expansion machine 30 that roll upon or glide along one another. Alternatively, the separation of the lubricant could be accomplished in the oil separator 10 substantially completely, and an adequate amount of lubricant could be recirculated into the live steam of the working medium before entering the expansion machine 30. The separated lubricating oil is collected in the oil separator 10. As it was brought to a high temperature after passing through the evaporator together with the working medium it is under a high pressure in the oil separator 10, allowing it to flow freely through a corresponding conduit to the expansion machine 30 so as to lubricate there corresponding lubricating points of same. For instance, the lubricant is provided in the working medium in a dissolved form when it is supplied by the feed pump 50 to the evaporator 20. In general, the boiling temperature of the lubricating oil will be clearly higher than that of the working medium, so that after passing through the evaporator 20 it will be present in the working steam of the working medium in a liquid form, viz. droplet form. As, according to the example described, the lubricating oil separated in the oil separator 10 is under a high pressure, allowing it to flow freely to the expansion machine 30 on account of the pressure, it is not necessary to provide another pumping device for the lubricant. Moreover, as compared with the previous state of the art, a smaller steam volume flows through the oil separator 10 per unit time so that same can be designed in a comparatively compact manner, resulting in the saving of space and a reduction of costs. In addition, the pressure loss is reduced downstream of the expansion machine 30 so that the pressure difference can be increased by means of the expansion machine 30, as compared with the conventional configuration comprising an oil separator 10 downstream of the expansion machine 30, thus allowing an efficiency increase of the expansion machine 30. Also, lubricant remains directly in the live steam of the working medium or is supplied to same at live steam temperature, respectively, so that in contrast to the previous state of the art the use of a lubricant does not result in a reduction of the live steam temperature and enthalpy.
In the operation of such a thermodynamic cycle it has shown, however, that the starting (start-up) is very difficult if the oil separator and the oil are cold. The operating temperature of the oil differs significantly from the downtime temperature. In operation, the oil has a temperature that is equal to the live steam temperature of approximately 100° C. However, during downtimes the oil temperature can go down to the ambient temperature, e.g. 10° C. to 25° C., or even down to minus centigrades. As the viscosity of the oil rises at such low temperatures by several orders of magnitude the start-up procedure of the cycle device is problematic. Although an electric heater could overcome this problem, additional investment costs and operating costs would be necessary. In addition, an electrical heating of the oil would take too long. Consequently, it is the object of the present invention to provide a method and a device for the rapid heating of the oil after a downtime of the cycle device described.